Tests carried out on humans suggest people with Alzheimer's disease could be diagnosed before symptoms start to appear, US doctors say.

Patients are often not diagnosed until the later stages, after brain damage.

But researchers said a sensitive test to detect a type of protein molecule could identify the disorder early after successfully trialling it on 30 people.

The team at Northwestern University in Chicago have claimed Alzheimer's could be treated if caught early.

Lead researcher William Klein said traditional diagnosis came too late for people with Alzheimer's.

This research is encouraging, but it is too soon to say how useful this particular technique will be as a diagnostic test

Harriet Millward, of the Alzheimer's Research Trust

"We think the accumulation of the molecules is likely to be the first biomarker in Alzheimer's disease, and now this extraordinary powerful detection technology has changed what we think might be possible."

One in 20 people over 65 years old and a fifth of those over 85 have Alzheimer's.

The team believes molecules, called ADDLs, are present at high levels in the cerebrospinal fluid at the onset of Alzheimer's and block memory function, the Proceedings of the National Academy of Sciences reported.

Using a highly sensitive test, which employs bionanotechnology, the team was able to check the levels of ADDL in 30 individuals and found they were consistently higher in people with the disease.

'Encouraging'

Researchers are now hoping to develop a blood or urine test to detect the molecules, as cerebrospinal fluid is more difficult to obtain.

Alzheimer's Research Trust deputy chief executive Harriet Millward said an accurate test for Alzheimer's in the early stages would help treatment, but she said there was still a long way to go.

"This research is encouraging, but it is too soon to say how useful this particular technique will be as a diagnostic test.

"The results will need to be validated in larger numbers of people.

"Taking cerebrospinal fluid from people is not very practical and can be painful, so scientists hope to develop the technology for blood or urine samples.

"It is also not yet clear whether this method will enable doctors to differentiate between Alzheimer's and other forms of dementia for which future treatments may differ."